Due to the predominantly explorative nature of this project (as outlined in the hypothesis component), the estimated sample size does not have the status of a strict cut-off, as is the case in clinical trials with more clear, confirmatory goals. Instead, the estimate functions more as a general guideline, indicating the sample size we should ideally aim for in each study, in order to have sufficient power to detect a medium-sized effect (when comparing Repetitive Negative Thinking and/or symptoms between conditions). As we have no access to data measuring the combined effects of Cognitive Control Training and group-based interventions (similar to Rumination-Focused Cognitive Behavioral Therapy and/or the fear of failure program) on Repetitive Negative Thinking, rules of thumb for the effect sizes had to be used for the simulations. GPower (Faul, Erdfelder, Lang, & Buchner, 1996) was used to calculate the sample size. A t-test for the difference between two independent means was selected, with alpha at 0.05, an 80% power level and a medium effect size (d = 0.50). For equally sized Cognitive Control Training (CCT) and Active Control Training (ACT) groups and no specification about the direction of the effect (i.e., two tailed test), a total sample of 128 participants is required. Optimal Design (Spybrook et al., 2011) leads to a similar result for an effect size (delta) of 0.50 and no covariates (single level, person randomized design). Given that the CCT is expected to outperform the ACT if there is indeed a difference, one could also select a one-sided t-test. With all else equal, the required total sample size decreases to 102, according to GPower. Additional simulations in Optimal Design were run to get an idea of the required sample sizes for different effect sizes, if a covariate (explaining varying amounts of variability) would be added. Graphs of all these analyses can be found in the storage section. This project aims to recruit at least 30 participants per condition, with the ambition to get as close to a total of 100 participants as possible. Both studies are planned to start in the second half of October (2017). The amount of participants will largely depend on the amount of patients in the group-based programs and the ability of the therapists to motivate their patients to sign up for the additional intervention. The project will run for a maximum of two years (until October 2019), even if the ideal amount of participants is not reached by then. *Faul, F., Erdfelder, E., Lang, A.-G., & Buchner, A. (2007). GPower 3: A flexible statistical power analysis program for the social, behavioral, and biomedical sciences. Behavior Research Methods, 39(2), 175–191. doi:10.3758/bf03193146* *Spybrook, J., Bloom, H., Congdon, R., Hill, C., Martinez, A., & Raudenbush, S. (2011). Optimal design plus empirical evidence: Documentation for the “Optimal Design” software Version 3.0. Retrieved on August, 4, 2017 from http://pj.freefaculty.org/software/WinProgs/od-manual-20111016-v300.pdf*